1. Field of the Invention
This invention relates to digital projection systems that project modulated laser beams onto a screen.
2. Description of Related Art
While cinemas of the past have relied almost exclusively upon moving pictures embedded in film copies, in the future cinema will likely become entirely digital. For example, digital movies will likely be distributed in a digital format such as a computer file. A digital image format ensures consistency over time because unlike film, digital images do not “wear down”. Furthermore, a digital format can be more easily combined with other information such as supplemental commercials or directed advertising. Generally, digital formats are a more cost-effective distribution method than film.
However, it has been difficult to develop practical and cost-effective projection systems. One such proposed alternative projection system utilizes a red laser, a green laser, and a blue laser, each of which are individually modulated in a series of “frames” and then projected onto a screen to create a full color image. In one such display architecture, the brightness distribution of the illuminating light includes brightness levels that follow a precise binary progression. Such systems, although theoretically feasible, are difficult to implement in the real world in a cost-effective manner. For example, in the above binary progression system it is necessary to create a discrete set of laser beams, each having an exact brightness relationship with the other. Such a binary progression of brightness levels is difficult to achieve in practice.
One obstacle to achieving a truly digital projector relates to the analog properties of many of the electrical, mechanical, and optical components used in a so-called digital system. For example, one known type of light modulator-a digital micromirror device (DMD)-operates by shuttering light using a two-dimensional array of extremely small mirrors. In principle, the length of time during which each micromirror is switched to the “on” position can be controlled in a precise matter. In practice, however a certain amount of time is required to transition between the “off” and the “on” states of each DMD micromirror. Although this transition time can be very short, the transition time can interfere with the gray level of the display (e.g. on the order of 10-20 microseconds) particularly when the image is “dark” such as when the requisite on-time becomes short.